


Light Me Up

by banbanabas



Category: Undertale (Video Game)
Genre: Alphynecentric, F/F, I CAN'T BELIEVE IM MAKING THESE TAGS HOLY SHIT, Oral Sex, Smut, Undertail, Underwater Sex, VERY AFRAID AUTHOR, bioluminescent undyne, remember when i said i wasn't going to write any smut, someone help me, very intrigued alphys, well here is some smut
Language: English
Status: Completed
Published: 2015-12-19
Updated: 2015-12-19
Packaged: 2018-05-07 14:24:43
Rating: Explicit
Warnings: No Archive Warnings Apply
Chapters: 1
Words: 3,310
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/5459684
Author URL: https://archiveofourown.org/users/banbanabas/pseuds/banbanabas
Summary: <blockquote class="userstuff">
              <p>It had been a long week, yes, and she knew the dedicated science instructor put in a lot of extra hours planning her lessons, but Undyne wanted to give her a chance to unwind, in her own way.</p>
<p>(Alternate title: SIN AND REGRET)</p>
            </blockquote>





	Light Me Up

**Author's Note:**

> For the record, I actually did some research about bioluminescence for this fic. I altered actual facts and chalked it up to Monster Biology. In case anyone was wondering.
> 
> Also, I haven't stopped blushing since I wrote this.
> 
> Happy holidays, sinners.
> 
> ALSO: for additional sin...... cattusinhortum . tumblr. com made some wonderful human au art of....... certain parts of this fic.........  
> http://cattusinhortum . tumblr . com/post/135593793649/n-nsfw-18-only-please-human-alphyne-scribbles  
> GOOD SHIT thank you for this gift i am wEAK

“Hey, Alphys,” Undyne mumbled. Pausing their show with a tap of the space bar, Alphys sleepily hummed her acknowledgment. “Wanna do something crazy?”

“Li’ what?” Alphys asked, voice muffled by her elbow.

“Let’s go to the beach.”

Alphys looked up incredulously at an unflinching Undyne. “A-are you serious? It’s almost midnight.”

Undyne leaned back on her elbows and sighed. “Yeah, but it’s a Friday! We don’t have to wake up early tomorrow, and I want to... Be outside, you know?”

The drawn-out noise Alphys emitted did nothing to discourage Undyne. “You’re outside a-all the time for soccer practice,” Alphys countered, exhaustion clouding her voice.

“That’s not the same thing. I want to relax. No obligations. No responsibility. Just you, me, the ocean, and the stars.” A blush crept onto Undyne’s face after those last few words escaped her.

“You... Oh, geez.” Alphys buried her face in her hands. “Th-that’s not fair. You can’t be all cute when you ask for stuff. B-because I can’t say no.” She shot Undyne an accusatory glance, but Undyne just grinned.

“Is that a yes?”

Alphys huffed. “I guess so.” Undyne whooped and nearly leaped off the mattress, launching Alphys and her laptop into the air behind her as she flung open the dresser drawers.

“Aw, man! Let’s go! Go put your swimsuit on, babe!” she shouted as she threw the garment in question to her girlfriend.

“W-wait, we’re, uh, swimming?”

Undyne bared her teeth in what was perhaps a predatory smile. “You don’t let a fish go to the beach without agreeing to get in the water.”

Alphys shook her head with a snort. “Heh. As long as you’re packing towels.”

///

The twenty-minute drive flew by. Undyne and Alphys had thrown on recreational clothes over their swimsuits, piled into their shared minivan (useful for transporting kids, equipment, and robotics tech) with a pile of towels and water bottles, and set off with Undyne at the wheel. She’d put on some upbeat music in a failed attempt to stifle Alphys’s yawns. It had been a long week, yes, and she knew the dedicated science instructor put in a lot of extra hours planning her lessons, but Undyne wanted to give her a chance to unwind, in her own way.

“We’re here,” Undyne announced. The parking lot was empty, unsurprisingly, and the two gathered their items and set out. They both froze when they reached the boardwalk overlooking the sandy dunes.

“Whoa,” Alphys whispered, and Undyne nodded in wordless agreement. The scene took her breath away. Hundreds of stars lit up the stark, cloudless sky, and the water looked almost perfectly still. She could see the moon’s shaky but beautiful reflection settled on the ocean’s surface several miles out.

“It’s beautiful,” Undyne finally managed to say. Alphys knocked a shoulder into Undyne’s arm.

“G-glad we came, Undyne,” she said. “It’s so p-pretty. There’re barely any lights around. Heh. P-perfect for seeing all the stars.”

If she hadn’t been holding their pile of towels, Undyne would have thrown an arm around Alphys, but she settled with nudging her shoulder in return. “I’m glad you let me talk you into it!”

They wandered onto the beach. Undyne removed her sneakers on the boardwalk and left them there. “Nobody’s gonna take ‘em,” she said. After a moment’s hesitation, Alphys kicked her sandals off as well and trudged hurriedly through the sand to catch up with Undyne. Near the shore, Undyne set down one towel, preparing the rest of the stack as pillows. They both laid back to admire the starlit sky.

They spoke quietly about their days, their weeks, what they planned to do and what they wished they could do. Undyne found herself peeking at Alphys’s face while she talked; she got a certain glint in her eye when she was excited about something. It appeared when she talked about her students’ robotics projects and Mettaton’s sudden success, and most importantly to Undyne, it appeared when she talked about her future. Warmth flooded her body at the thought, and as a blush appeared on her cheeks, Alphys sneaked a glance at Undyne. “What?” she asked.

“Nothing,” Undyne said, turning her gaze to her fingers as she picked at a loose thread. “Just... Happy.” Alphys hummed.

“M-me, too.” She placed a hand over Undyne’s between them, and Undyne couldn’t help but smile.

After a few moments of indulgent, dorky sappiness, Undyne sat up. “Wanna go for a dip?”

Alphys followed suit, though more slowly than her excitable partner. “Do I have a choice?” she asked jokingly.

“Yes,” Undyne said, “of course you do. But if you don’t want to, you won’t get to see me in my swimsuit.”

Alphys paused, her lips pursed, and then said with conviction: “L-let’s swim.” Undyne laughed as she jumped to her feet and offered a hand to Alphys.

“Knew that would get you,” she said slyly, relishing in Alphys’s blush. While she still had the smaller woman’s attention, she leaned over, oh-so-slowly dragged the shorts down her legs, and kicked them away. She made eye contact with Alphys as she stood again to do the same with her T-shirt. Oh, yes, the blush she got for _that_ one was even better.

“Uh...”

Undyne smirked. “You coming?”

Alphys shook herself and forced her eyes back up to Undyne’s. “Um. Uh, uh-huh.”

“Then strip,” Undyne quipped, winking, as she sauntered off in her two-piece to meet the waves. Though she wanted to see Alphys’s reaction, she decided against it in favor of the dramatic exit, but she still had to purse her lips tightly to contain the resulting giggle.

But, oh, man. The water felt great. The late summer sun had instilled enough heat to keep it from chilling too badly. Undyne took several assured steps into the tide and breathed the salty air; this was her element. When she was about thigh deep, she heard sloshing behind her that meant Alphys had finally followed. She slowed her steps to let the shorter woman catch up.

“Undyne?” Alphys called. Smiling deviously, Undyne spun around and gathered her in her arms, then sunk in the waves so that only their faces were unsubmerged. Alphys squeaked in surprise, and Undyne pressed a kiss onto her cheek.

“Cute.” Holding her tightly, Undyne kicked off from the sand and began swimming backwards, one hand churning water while the other kept Alphys secure. Undyne’s webbed appendages easily kept their combined weight afloat. Alphys, for her part, clung to Undyne with both arms like a marsupial’s surprised offspring.

“I c-can swim, you know,” she muttered halfheartedly.

“I know. But tonight, I’m the S. S. Undyne, and I’m going to ferry you around.” Alphys snickered.

When they were deep enough for Alphys to stand with only her head above the water, Undyne stopped kicking and let her go free. Alphys looked up at her questioningly, but before she could ask, Undyne mumbled, “I have something to show you.”

“What?” Without replying, Undyne closed the distance between them and cupped Alphys’s face in her hands. They stared into each other’s eyes a moment, Alphys in surprise and Undyne in anticipation, and then Undyne gently pressed her lips against Alphys’s. She tilted her head teasingly slowly, moving her lips just enough to elicit a mirrored response in Alphys. When she pulled back, Alphys gasped. “That was... Oh! What! You’re… You’re all l-lit up!”

Undyne looked down, now surprised herself. “Whoops! Ha ha, usually it takes longer than that,” she said under her breath. The skin on her cheeks, arms, chest, midriff, and legs were covered in speckles that glowed faintly blue. She grinned at Alphys. “Cool, huh?”

“How are you doing that?” she questioned, extending a claw toward Undyne but then hesitating. “Can I touch?”

“Sure.” She moved an arm under Alphys’s hand. Alphys traced careful lines between the glowing spots, and Undyne suppressed a shiver. “I’m bioluminescent. It only works underwater when I have—” She laughed sheepishly. “—increased blood flow.”

Alphys’s hand froze. “Oh. Heh.” She looked up at Undyne with a knowing look, and Undyne’s fins twitched upward. Alphys placed her other hand on Undyne’s shoulder and drew lazy circles with a claw, her eyes still focused on Undyne’s. She did shiver, this time.

They wandered the waves for a while. Alphys stared unabashedly at her lover’s gleaming skin, completely enthralled, and they exchanged kisses every now and then. The harder she kissed her, the more Undyne glowed. Alphys felt a twist of pride when a particularly well-timed bite on Undyne’s shoulder resulted in a flash of blue light.

Some time later, Undyne offered to carry Alphys out deeper. “You can sit on my shoulders while I walk,” she explained, pointing to her gills. Alphys nodded.

“If you g-get uncomfortable or tired, you can... Hm. Tap my ankles. And I’ll get off you,” Alphys offered.

“Gotcha.” Undyne dipped below the waves, her ponytail wavering in the slight pull of an undercurrent. She slid between Alphys’s legs, closed her hands around her ankles, and pushed off from the sand for deeper waters.

Alphys giggled like a schoolgirl. It felt like swimming and walking at the same time, and she could still see Undyne’s glow from above the waves. Initially, Alphys had worried about balancing, but Undyne leaned her upper body into the surf so that Alphys was stable without having to wave her arms around or clench her thighs around Undyne’s head.

Undyne, meanwhile, was having a similarly enjoyable time. Underwater, her hearing and sense of smell were dulled in favor of touch-based senses. She could feel the vibrations in the water around her, most stemming from fish that appeared and disappeared in her peripheral vision, too afraid to hang around her personal light show but too curious not to give a peek. Tearing her eyes from the flickering shadows, Undyne kept a gentle hold on Alphys’s ankles and focused on walking. It felt a little odd to sink her feet in the sand while her whole body was submerged. Odd, but not uncomfortable.

Speaking of uncomfortable... Or, perhaps, not quite uncomfortable, but noticeable...

Undyne was starkly aware of her head’s location in reference to the rest of Alphys. And it was driving her just a little nuts. (And maybe Alphys was aware of the very same thing, but she sure wasn’t going to draw attention to it because she was enjoying this evening and did _not_ want to make it weird. Not in the slightest.) Every time Alphys shifted, Undyne noticed. She noticed very much.

And then, when Undyne stopped moving for a second to change directions, Alphys pitched forward a bit, and her groin pressed against the back of Undyne’s head. That would have been fine other than Undyne’s sudden blush, but she felt Alphys lean into her a little more than necessary. Almost like she... Meant to increase the pressure there.

She couldn’t take it anymore.

Undyne’s hands traveled from Alphys’s ankles to her thighs, lifting her slightly so Undyne could twist her body and swim upward. She hit the surface just in front of Alphys’s surprised face. “I-is something wr—” she began, but Undyne, still treading water, pulled her closer by the waist and kissed her roughly. Alphys barely hesitated before reciprocating, throwing her arms over Undyne’s shoulders to bring her ever nearer. Undyne arched her back to press their chests together, and Alphys broke the kiss to gasp. “Oh, my G-God...”

Undyne didn’t waste any time. “I want you,” she rasped. Alphys stiffened.

“R-right now? Here?” she asked quickly. Undyne pressed several kisses down her jaw. “In... In public?”

“I don’t care if anyone sees,” she said into Alphys’s neck. “I want you right here, right now.” She pulled back for a second to search Alphys’s face. She was flushed beet red, but her eyes were obviously dilated. “If you’re okay with it.”

“I...” Alphys bit her lip. “I... Am... M-more th-than okay with it?” she responded shakily. They bobbed in the water, up and down in time with Undyne’s kicks. Undyne lifted a hand from the water to cup her face.

“You can tell me if you don’t want to,” she said, kissing her forehead. “It’s okay. I know this isn’t exactly, ah ha ha, our normal thing.” Alphys laughed nervously.

“No, hey, I j-just...” She smiled at the surface of the water. “I think I... W-wanted to. Before you said so. And I w-was surprised by the, heh, offer.”

Undyne grinned and kissed her with parted lips, pulling her closer by the back of her neck. When she released, she looked Alphys in the eye again and asked, “So do I have permission?” with a cursory glance downward. Alphys nodded.

“Oh, m-my God, yes. Yes!”

Grinning toothily, Undyne dove below the surface and placed Alphys’s legs where they’d been before, but this time she hooked her knees over her shoulders while she faced into Alphys. Undyne traced her claws up the outsides of her thighs and then gripped her by the waist. The fabric of her one-piece slipped a little under her palms. With a smile that would rival a shark’s, Undyne placed one hand on the curve of Alphys’s rear (She squeaked above the waves.) to hold her aloft while the other caressed her inner thighs (She took a deep, shaky breath.). Placing kisses wherever her fingers grazed, she inched closer and closer to Alphys’s groin (She bit her hand to stifle various other noises.) until the only thing separating her from Alphys was two inches of water and a thin strip of nylon. She pressed a gentle kiss onto the fabric (She cried out.) and, with careful fingers, pulled it aside.

Perhaps something was different about seeing her underwater, or maybe it was how she looked in the light of Undyne’s self-sustaining, sapphire glow. Regardless, Undyne swore this was the best view of her she’d ever had.

And then she set to work. She laid a kiss against the exposed yellow skin and felt a hand on her scalp. She slowly edged her tongue out; the hand tightened in her hair. She traced a line teasingly slowly down to Alphys’s lips, and she felt the hand make a fist and pull. Demanding, wasn’t she? Might as well give her what she wanted.

Undyne licked long and slow, back to front, once, and Alphys shuddered harshly against her. She repeated the motion, twice, thrice, and with every lingering touch of her tongue against her center, Alphys’s thighs tightened around her head. She pinched Alphys’s rear. The legs upon her shoulders clenched worryingly tightly, and if Undyne had had the space to do so, she would have laughed.

But the lack of space was getting... Troublesome. The water near Undyne’s gills wasn’t providing quite enough oxygen, and the obvious solution was to free up the space near her gills. Undyne had an idea of just how to do that.

She trailed both hands along the outsides of Alphys’s thighs, over, and then down her inner thighs as far as she could reach, allowing her a firm grip around each leg. Her tongue continued its languid movements while her claws traced circles on the delicate skin. Soon, Undyne began to press gently outward with both hands, effectively spreading Alphys’s legs—just enough for her to breathe better, and apparently, just enough to make Alphys moan into the hand that wasn’t tangled in Undyne’s hair.

Undyne returned her focus to Alphys’s center, broadening her tongue against the outer lips as she slid it upward. Alphys seemed to understand the situation with Undyne’s gills because she resisted the urge to clench her thighs, instead curling her torso forward and digging her claws into Undyne’s scalp.

Without warning, Undyne flicked her tongue against Alphys’s clit, and Alphys cried out, grinding herself against Undyne’s face. Undyne smirked, repeated, flitting her tongue against her sweet spot to make her squirm. When a second fist joined the other in her hair and ripped her ponytail holder away, she decided it was time to get serious. Shifting her head a bit lower, Undyne dipped her tongue between her folds and pressed _in_. Alphys ground against her again, a little more aggressively, so Undyne dug her tongue deeper, more firmly, over and over until the saltiness of the sea disappeared beneath the taste of her lover. Alphys continued to quiver, locked in a half-crescent over Undyne’s head with her claws working against her scalp (She was whimpering Undyne’s name, too, but Undyne wouldn’t know that.).

In one smooth motion, Undyne moved her tongue back to her clit, removed her right arm from Alphys’s thigh, and positioned that hand on her backside. The now-freed leg curled over Undyne’s bicep so that one very stiff heel was pressed into her back. In response, Undyne trailed a finger to Alphys’s entrance, pressed lightly, and paused. Alphys wiggled her hips downward. Undyne grinned... And pushed.

To maintain her dignity, Alphys dipped her snout below the water to muffle the ensuing moan, but... Well, Undyne seemed to know exactly what a moaning lizard’s sound vibrations would feel like because she pressed her tongue even harder and twisted her finger just so. Alphys twitched harshly, driving her hips down on Undyne’s finger as much as she could from her position, and Undyne mercifully began to thrust. Breathless, Alphys rose from the water, panting and shaking and holding on tight to the woman who was making her an inarticulate mess. Undyne added another finger, and twisted, all the while still giving close attention to her clit.

And finally, when Undyne thrust in deep and curled her fingers, Alphys keened and tensed her whole body. Undyne quickened her movements as Alphys dragged the claws through her hair aggressively, jerkily grinding herself against Undyne’s tongue and fingers, her heel digging into Undyne’s back to pull her closer.

When the waves of heat retreated from her body, Alphys sighed, relaxed, and laid back in the water to disentangle herself from Undyne. Losing her grip on the limp woman’s legs, Undyne kicked off from the seabed and emerged beside her at the surface. Immediately, Alphys cupped her face and kissed her so passionately that Undyne forgot to tread water, and they dipped below the waves, lost in each other, until Undyne regained her senses long enough to bring them up so Alphys could breathe. Once she had Alphys cradled in her arms, Undyne began the short trip back to shore.

“That was _amazing_ ,” Alphys whispered lowly. Undyne grinned and placed a kiss on her forehead.

“ _You’re_ amazing.”

Once they reached standing height, Undyne released Alphys, but she stumbled and nearly fell headfirst into the waves. “I... Heh. Whoa. M-my legs are j-jelly, right now.”

“I got you, babe.” Undyne swept her up again and carried her to their spot in the sand. She wrapped her in a dry towel, grabbed one for herself, and sat beside Alphys’s blissful, unmoving form. Alphys rearranged herself so that her head rested in Undyne’s lap. The latter’s skin had dulled in the cool evening air.

“Hey,” she whispered.

“Hey.”

Alphys lifted a hand from the towel and touched Undyne’s neck. “I owe you one.”

Undyne smirked mischievously but snorted. “Yeah. But you don’t have to right now. We’re kind of, you know, actually visible right here.” Undyne looked down and winked. “But there’s always time for sushi when we get home.”

Alphys stared up at her for a second before it dawned on her. “Oh, my God, you d-did _not_ just say that!”

Undyne cackled. Before she could follow up with another joke, Alphys shushed her and said, “I swear, if you call it that again, I am going to die. Of embarrassment.”

Undyne kissed her forehead. “Nah. I’m hilarious, and you love it.”

Alphys sighed. “I do love you.” And then she added: “You dork.”

“Love you too, nerd.”


End file.
